Hot melts are generally understood to be room temperature-solid to waxy and solvent-free adhesives that are applied from the melt onto the parts to be glued together and after assembly physically set by solidifying on cooling.
Hot melt pressure sensitive adhesives (HMPSA) belong to the group of hot melt adhesives, remain permanently tacky and adherent on cooling and with slight contact pressure stick immediately to almost all substrates.
Hot melt pressure sensitive adhesives are widely used in industry and are employed for example in the beverage industry for labeling, in the packaging industry, in the graphic arts industry, for example in bookbinding, or for manufacturing disposable articles, for example, articles of hygiene, such as baby diapers.
The critical advantage of pressure sensitive hot melt adhesives is that they allow a solventless application. Thus, cost intensive intermediate drying steps are obviated. As solvent-containing adhesive systems are obliged to conform to increasingly more stringent environmental regulations, hot melt pressure sensitive adhesives also offer advantages from the ecological point of view.
Hot melt pressure sensitive adhesives based on polyacrylates are known. Thus, WO 02/10307 describes pressure sensitive adhesive compounds that consist of block copolymers, e.g., copolymers of acrylates, isoprene and styrene, for the manufacture of adhesive tapes. In addition, in one embodiment a component is comprised of an unsaturated group that is suitable for radiation chemical crosslinking. These pressure sensitive adhesive compounds crosslink by means of radical polymerization of olefinic double bonds.
In WO 00/027942, radiation curable pressure sensitive hot melt adhesives are described for coating labels at low temperature. However, these adhesives are semi-liquid at room temperature and are crosslinked by radiation curing after application. This involves a radical crosslinking of olefinically unsaturated polymers.
WO 00/22062 describes radiation crosslinkable adhesives that comprise block copolymers containing at least one butadiene block with a 1,2-vinyl content of at least 25%. As a result of the presence of olefinic double bonds, these adhesives can likewise be crosslinked by photo initiator radiation curing.
Radiation curable adhesives that can be applied at low temperature are described in WO 01/55276. They comprise block copolymers, wherein one block is formed from polyvinyl-aromatic building blocks and a second block is a polydiene block with vinyl functionality.
Furthermore, EP 1130070 describes radiation crosslinkable adhesives for coating heat shrinkable films or labels. These adhesives can consist of epoxidized block copolymers and/or cycloaliphatic epoxides with olefinic double bonds, a photo initiator, a tackifying resin, as well as further additives. Moreover, polymers can include olefinic double bonds or even unreactive copolymers based on isoprene or isobutylene. Crosslinking through crosslinking groups such as epoxy groups or through double bonds is described. A more exact limitation of the epoxidized block copolymer is not provided.
Disadvantages of the prior art are that for specific applications, pressure sensitive hot melt adhesives with the lowest possible viscosity in processing conditions are not yet available. This property is predominantly influenced by the choice of the polymer for the hot melt adhesive. A further disadvantage of hot melt adhesives according to the prior art is an inadequate initial adhesion with concomitant high heat stability of the joint, which is required for certain application purposes.